The changes in the activity and content of sucrase-isomaltase complex (S-I) in the intestinal mucosa were studied during the development of diabetes induced by streptozotocin in rats. On days 0, 1, 3, 5, and 10 after an intraperitoneal injection of streptozotocin (70 mg/kg), the enzyme activity and the enzyme content were observed in the jejunum and ileum. Sucrase and isomaltase activities markedly increased from the 3rd day both in the jejunum and ileum, and kept increasing till the 10th day especially in the ileum. The enzyme content of S-I also increased in parallel with its activity during the development of diabetes. However, in the early stage of diabetes, sucrase activity per microgram of S-I content increased both in the jejunum and ileum. Isomaltase activity per microgram of S-I content increased temporarily in the ileum. These results suggest that the increase of disaccharidase activities in the early stage of diabetes induced by streptozotocin is not only due to the increase of the enzyme content, but also due to the change of the enzyme catalytic property.